READ(2) NetBSD System Calls Manual READ(2)
NAMEread, readv, pread, preadv -- read input
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <unistd.h>ssize_tread(int d, void *buf, size_t nbytes);
ssize_tpread(int d, void *buf, size_t nbytes, off_t offset);
#include <sys/uio.h>ssize_treadv(int d, const struct iovec *iov, int iovcnt);
ssize_tpreadv(int d, const struct iovec *iov, int iovcnt, off_t offset);
DESCRIPTIONread() attempts to read nbytes of data from the object referenced by the
descriptor d into the buffer pointed to by buf. readv() performs the
same action, but scatters the input data into the iovcnt buffers speci-
fied by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
pread() and preadv() perform the same functions, but read from the speci-
fied position in the file without modifying the file pointer.
For readv() and preadv(), the iovec structure is defined as:
struct iovec {
void *iov_base;
size_t iov_len;
};
Each iovec entry specifies the base address and length of an area in mem-
ory where data should be placed. readv() will always fill an area com-
pletely before proceeding to the next.
On objects capable of seeking, the read() starts at a position given by
the file pointer associated with d (see lseek(2)). Upon return from
read(), the file pointer is incremented by the number of bytes actually
read.
Objects that are not capable of seeking always read from the current
position. The value of the file pointer associated with such an object
is undefined.
Upon successful completion, read(), readv(), pread(), and preadv() return
the number of bytes actually read and placed in the buffer. The system
guarantees to read the number of bytes requested if the descriptor refer-
ences a normal file that has that many bytes left before the end-of-file,
but in no other case.
RETURN VALUES
If successful, the number of bytes actually read is returned. Upon read-
ing end-of-file, zero is returned. Otherwise, a -1 is returned and the
global variable errno is set to indicate the error.
ERRORSread(), readv(), pread(), and preadv() will succeed unless:
[EBADF] d is not a valid file or socket descriptor open for
reading.
[EFAULT] buf points outside the allocated address space.
[EIO] An I/O error occurred while reading from the file sys-
tem.
[EINTR] A read from a slow device (i.e. one that might block
for an arbitrary amount of time) was interrupted by
the delivery of a signal before any data arrived. See
sigaction(2) for more information on the interaction
between signals and system calls.
[EINVAL] The file pointer associated with d was negative.
[EINVAL] The total length of the I/O is more than can be
expressed by the ssize_t return value.
[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.
In addition, readv() and preadv() may return one of the following errors:
[EINVAL] iovcnt was less than or equal to 0, or greater than
{IOV_MAX}.
[EINVAL] One of the iov_len values in the iov array was nega-
tive.
[EINVAL] The sum of the iov_len values in the iov array over-
flowed a 32-bit integer.
[EFAULT] Part of the iov points outside the process's allocated
address space.
The pread() and preadv() calls may also return the following errors:
[EINVAL] The specified file offset is invalid.
[ESPIPE] The file descriptor is associated with a pipe, socket,
or FIFO.
SEE ALSOdup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), sigaction(2),
socket(2), socketpair(2)STANDARDS
The read() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). The
readv() and pread() functions conform to X/Open Portability Guide
Issue 4, Version 2 (``XPG4.2'').
HISTORY
The preadv() function call appeared in NetBSD 1.4. The pread() function
call appeared in AT&T System V.4 UNIX. The readv() function call
appeared in 4.2BSD. The read() function call appeared in Version 6 AT&T
UNIX.
CAVEATS
Error checks should explicitly test for -1. Code such as
while ((nr = read(fd, buf, sizeof(buf))) > 0)
is not maximally portable, as some platforms allow for nbytes to range
between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
error-free read() may appear as a negative number distinct from -1.
Proper loops should use
while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
NetBSD 5.0_RC4 August 2, 2007 NetBSD 5.0_RC4

You can also request any man page by name and (optionally) by section:

Command:

Section:

Architecture:

Collection:

Use the DEFAULT collection to view manual pages
for third-party software.